1. Field of the Invention
This invention relates to a method and apparatus for molding plastic products, and, more in particular, to a method and apparatus for molding plastic products, which are suitable for manufacturing plastic products of complicated configuration and/or which are suitable for manufacturing sheet-type products with increased efficiency.
2. Description of the Prior Art
It has been common practice to fabricate products such as a pipe to be used as an air duct for a ventilation or cooling device, or an inner surface cover of the door of an automobile, etc. from thermal plastic resin materials like polyethylene, polypropylene and the like. In fabricating a plastic pipe of thermal plastic resin, blow molding may be advantageously employed, but if a pipe to be fabricated is complicated in configuration with acute bends and/or sudden area changes, then, there is a problem of sticking of opposing inside surface portions of a parison when placed in a mold cavity. The prior art technology did not propose any solution to this problem so that there was a limitation in complexity and size of a plastic tube to be manufactured by blow molding.
It has also been common practice to employ the conventional vacuum forming technology if one was interested in fabricating sheet-type plastic products. In accordance with the vacuum forming technology, a plastic sheet material is placed on a mold having a defined shape and, upon application of heat, the sheet material is drawn down into the mold by vacuum thereby fabricating a sheet-type plastic product having a desired shape. However, this prior art technology is disadvantageous in that it requires trimming after shaping which is quite time-consuming. Moreover, the larger the surface area of and/or the thinner the thickness of a product to be fabricated, the more difficult to apply vacuum forming with requiring a large capacity vacuum unit. It is also to be noted that the conventional vacuum forming technology basically provides for fabrication of sheet-type products one at a time.